Methylated DNA has been studied as a potential class of biomarkers in the tissues of most tumor types. In many instances, DNA methyltransferases add a methyl group to DNA at cytosine-phosphate-guanine (CpG) island sites as an epigenetic control of gene expression. In a biologically attractive mechanism, acquired methylation events in promoter regions of tumor suppressor genes are thought to silence expression, thus contributing to oncogenesis. DNA methylation may be a more chemically and biologically stable diagnostic tool than RNA or protein expression (Laird (2010) “Principles and challenges of genome-wide DNA methylation analysis” Nat Rev Genet 11: 191-203). Furthermore, in other cancers like sporadic colon cancer, methylation markers offer excellent specificity and are more broadly informative and sensitive than are individual DNA mutations (Zou et al (2007) “Highly methylated genes in colorectal neoplasia: implications for screening” Cancer Epidemiol Biomarkers Prev 16: 2686-96).
Nucleic acids from stool samples that are analyzed for the presence of mutations and/or for methylation status associated with disease or risk of disease typically pass through a number of process steps during analysis. These steps may comprise, e.g., filtration, precipitation, capture, washing, elution, and/or chemical modification. For analysis of DNAs to determine methylation status, e.g., the percent methylation of a test DNA, processing typically comprises treatment with bisulfite to convert un-methylated dC bases to dU residues, making them more readily distinguishable from the methyl-C residues that are protected from bisulfite conversion.
Accurate quantitation of a test DNA (e.g., determining percent methylation, presence and amount of DNA carrying a mutation, etc.) typically requires normalization to a control nucleic acid, e.g., an endogenous invariant gene having known features (e.g., known sequence, known copy-number per cell). Normalizing controls for sample-to-sample variations that may occur in, for example, sample processing, assay efficiency, etc., and allows accurate sample-to-sample data comparison.